Programs having a variety of contents are broadcast everyday without minding watchers' concern. For example, some of the TV programs contain scenes of violence or sex that parents do not want their children to watch.
Some receivers such as television receivers that receive programs have a function of restricting the viewing of undesirable programs by muting their images and sounds.
However, the television receivers having such a restricting function require those who supervise children to perform bothersome operations such as setting the channel and broadcast time of a program of which viewing is to be restricted. Moreover, the setting must be made for each program at a time; therefore, the setting is often forgotten allowing children to watch undesirable programs.
To solve such a problem, a method of restricting program viewing is proposed in which a program rating (hereinafter also referred to as rating data) offered by the extended data services (XDS) that has been provided in the United States for example is used. The rating data are associated with program rating; that is, the rating data classify television programs according to their contents. For example, programs are classified in accordance with a viewer's age. Programs are classified into those for any age group and others for particular age groups, viewers over 13 years old for example.
In XDS, data (data transmitted by XDS are hereinafter referred to as XDS data as appropriate), in particular the rating data, are superimposed on line 21 (the 21st horizontal scanning line) of the second field (the even-number field) in a vertical blanking interval of a television signal based on NTSC (National Television System Committee) to be transmitted with a program. Details of XDS are specified in EIA (Electronic Industries Association)-608.
A flowchart of FIG. 8 indicates an example of a method of viewing restraint by use of the rating data transmitted with a program. First, the transmitted program is received in step S41 to be subjected to necessary processing including demodulation. The image and audio signals constituting the received program are output. In step S42, line 21 of the second field is sliced from the image signal to extract the XDS data superimposed on the line 21.
Then, in step S43, it is determined whether the XDS data are rating data or not. If the XDS data have not been found the rating data, the processing goes back to step S41. If the XDS data have been found the rating data, namely, the rating data have been received, the processing goes to step 844. This step determines whether the rating data are higher than a predetermined viewing restraint level or not.
Here, it is assumed that the rating data provide a value that is suitable to an age group for which viewing of a particular program is recommended. To be specific, the rating data of a program for age groups of 17 years old and higher is higher than the rating data of a program for age groups of 13 years old and higher for example and the rating data of a program for adults is higher than the rating data of a program for age groups of 17 years old and higher for example. Consequently, in this case, if received rating data are higher than the rating data of the program for 13 years old and higher, it is not desirable to allow the children under 13 years old (or broadly, around 13 years old) to watch the program that has been received along with the received rating data.
The predetermined viewing restraint level can be set by guardians such as parents. For this level, rating data of programs that parents do not want their children to watch may be set for example.
In step S44, if it is determined that the rating data are higher than the viewing restraint level, the processing goes to step S45. In step S45, the viewing of the program transmitted along with the rating data is restrained, or the image and audio signals of the program corresponding to the rating data are muted, upon which the processing goes back to step S41. Therefore, in this example, the rating data become higher as age groups become higher, so that, if the data rating has been set to a level for a program allowed to viewers of 13 years older and higher, the viewing of the programs for those age below 13 is all restrained.
On the other hand, if it has been determined in step S44 that the rating data are lower than the viewing restraint level, the processing goes to step S46. If the viewing has been muted in step S45, the muting is cleared and the processing goes back to step S41. Namely, if the viewing of a program transmitted on a certain channel for example was restricted in step S45 and then transmitting of a program having rating data lower than the viewing restricting level has started upon ending of the restricted program, the restraint of the viewing is cleared.
According to the above-mentioned method, setting viewing restraint levels alone allows the guardians of children for example to perform TV program viewing restraint.
Meanwhile, the XDS data offered by XDS provide program advertisement when the XDS data are used by viewers for presentation and the like, so that TV stations transmit as the XDS data not only the above-mentioned rate data but also data including program type, title, program identification number (program start identification number), program length, and program time-in-show.
The XDS data containing the above-mentioned data are transmitted in packets. In particular, the rate data are transmitted in the following packet of six bytes:                01h (start control for XDS);        05h (current class type);        rating data 1 (rating byte 1);        rating data 2 (rating byte 2);        checksum code; and        end code.        
In the above-mentioned packet, 01h (h denoting hexadecimal notation) indicates a class to which the rating data belong (namely, the current class) and the start of the packet and 05h indicates that the packet is of the rating data. The rating data 1 and 2, checksum code, and end code are one byte long each.
It should be noted that this packet can have two-byte data of the rating data 1 and the rating data 2; however, according to EIA-608 as of Sep. 20, 1994, the rating to be used is only the rating data 1, the rating data 2 being null.
Also, the other XDS data mentioned above are transmitted in a packet of similar type.
Further, when transmitting a packet of rating data continuously at a time, the data length of the packet is equal to the length of the packet, namely six bytes; if the packet cannot be transmitted continuously, the packet is divided for transmission. In this case, a continue byte is transmitted to indicate that the divided portions constitute one packet, making the data length greater than six bytes.
In XDS, in addition to the transmission of each piece of XDS data in the above-mentioned packet, XDS data of a plurality of types may be combined to be transmitted in a composite packet. The combinations to be arranged in a composite packet 1 include the following for example:                program type;        program rating;        length;        time-in-show; and        title.        
The program type is five bytes long, the rate data is one byte long, the length is two bytes long, and the time-in-show is two bytes long. The title is zero to 22 bytes long.
A following combination of XDS data is also possible as composite packet 2 for example:                program start identification number;        audio services;        caption services;        call letters;        native channel; and        network name.        
The audio services represent the contents of main audio and second audio, while the caption services represent the contents of teletext. The call letters indicate call letters of local stations. The native channel indicates the channel of a local station. The network name indicates a station network to which the local station belongs. The program start identification number is four types long; the audio services are two bytes long; the caption services are two bytes long; call letter are four bytes long; and the native channel is two bytes long. The network name is 0 to 18 bytes long.
The bandwidth of the XDS data transmitted by XDS is higher than closed captioning data in priority, thereby limiting the bandwidth. To be more specific, the line 21 of the second field can be superimposed with two-byte data, so that its bandwidth is 60 bps (bit per second) (=2 bytes times 30 (the second field/second) but the bandwidth for XDS is limited to less than 30% of the above-mentioned bandwidth as specified in the EIA-608. Consequently, the maximum bandwidth for XDS data is about 20 bps.
If only the composite packets 1 and 2 mentioned above are taken for example, their data amount to 24 to 64 bytes. Therefore, when the bandwidth is 20 bps, the transmission of only the composite packets 1 and 2 takes 1.2 seconds to 3.2 seconds. Further, XDS data are telecast not only from a single station but also a plurality of stations, so XDS data transmission takes more than the above-mentioned times.
Hence, a period in which the rating data arranged in the composite packet 1 are received on the received side becomes fairly long (even when only the composite packets 1 and 2 are transmitted from one station, the rating data receive period becomes 1.2 second).
Meanwhile, in the viewing retraining method described with reference to FIG. 8, rate data are received, viewing restraint is performed and viewing restraint is cleared based on the received rating data. Therefore, if a program on a certain channel is to be restrained, switching from that channel to another viewable channel and then to the first channel again does not restrain that channel until its rating data come, allowing the program that is to be restrained to be viewed. Thus, switching channels in a short period causes the viewing restraining capability to be disabled substantially (it should be noted that, in XDS, data for automatically controlling receiving equipment may be transmitted, also causing this problem).